1. Field of the Invention
The invention relates generally to circuits for measuring electric current, and, more particularly, to circuits for measuring abnormally high or low phase currents, and/or ground overcurrent in a three-phase electric power circuit.
2. Prior Art
U.S. Pat. No. 4,007,401, issued Feb. 8, 1977, to Kimmel et al., discloses a well known six-path, three-phase, full wave rectifier bridge having its A.C. inputs supplied from three wye-connected current transformers coupled to respective phase lines of an electric power circuit, and a load resistor connected across its D.C. output. When used to measure current in an ungrounded three-phase, three-conductor circuit in which the three conductors are the only current paths, the D.C. voltage across the load resistor will be an accurate, proportionate measure of the sum of the positive polarity currents supplied by the current transformers, and thus a proportionate measure of the sum of the phase currents in the electric power circuit. However, this current measuring circuit is not suitable for three-phase circuits having a fourth current path, or the possibility of such, such as a grounded wye-connected system, in which the three-phase currents do not necessarily cancel. Since the only return current path to each current transformer is through the other two current transformers, when these three currents do not cancel, these current transformers will buck one another, and quickly saturate. As a result, the current measurement is inaccurate, and excessively high voltages can be generated across these current transformers.
In other known bridge circuits for measuring phase overcurrents in any type of three-phase circuit, three individual four-path, single phase, full wave rectifier bridges, each having its A.C. input supplied from a respective one of the three current transformers, are used to produce a D.C. output signal proportional to a respective phase current of the power circuit. The D.C. outputs of these individual single phase bridges can be connected in series, as shown in U.S. Pat. No. 3,689,801, issued Sept. 5, 1972, to Engel et al., or in parallel, as shown in U.S. Pat. No. 3,573,555, issued Apr. 6, 1971, to James W. Lipnitz, across a load resistor to produce a peak voltage across the resistor proportional to the highest value of current in any of the three phase lines. However, these measuring circuits cannot be used to measure a minimal value of current through any one of the three phase lines, which could be used, for example, to detect an open line condition, and trip a power line circuit breaker to thus prevent single phasing of the three-phase motor loads connected downline from the circuit breaker.
In known bridge circuits for producing a D.C. voltage proportional to the ground or neutral current in a three-phase, four-conductor, wye-connected power circuit, generally three current transformers are used in addition to the three current transformers used for measuring the phase currents, as disclosed in my U.S. Pat. No. 4,027,203 issued May 31, 1977. Alternatively, the same three current transformers can be used for both phase and ground overcurrent detection, by using additional transformers between the three current transformers and the individual phase or ground rectifier bridges. For example, each current transformer can be connected to the primary winding of a respective phase transformer, in series with a respective one of three primary windings of a ground transformer, with the secondary winding of each phase transformer and the ground transformer being connected to a single phase, full wave rectifier bridge, as disclosed in U.S. Pat. No. 3,803,455 to Henry G. Willard, issued Apr. 9, 1974.